Flexible electronic card and method

ABSTRACT

A flexible electronic card for use with an electronic card reader including a flexible substrate formed of plastic. The card has dimensions such that it can fit into a conventional billfold. A flexible semiconductor device is carried by the flexible substrate and is accessible electronically by the electronic card reader. The card and the semiconductor device carried thereby can withstand bending over a 2&#34; radius without breaking or damaging the semiconductor device.

This is a continuation of application Ser. No. 08/415,185 filed Apr. 3,1995 now abandoned.

This invention relates to a flexible electronic card and a method formanufacturing the same.

Electronic cards often called Smartcards have heretofore been provided.Because of the increasing electronic requirements for such cards it hasbeen necessary to use larger and larger semiconductor devices to performthe necessary electronic functions for the card. Typically thesemiconductor devices utilized in such cards are formed of a rigidmaterial as for example of silicon which resist bending and have atendency to crack and break when the cards are bent by the user. Thereis therefore need for a new and improved flexible electronic card whichcan incorporate therein large semiconductor devices and in which thesemiconductor devices can withstand severe punishment and will not breakor fracture during the lifetime of the card.

In general, it is an object of the present invention to provide aflexible electronic card which can withstand the bending normallyencountered during usage of the card without breaking or damaging thesemiconductor device carried therein.

Another object of the invention is to provide a card and method of theabove character in which the card and the semiconductor deviceincorporated therein can withstand bending over a radius of 2" or lesswithout damage to or breaking of the semiconductor device carriedtherein.

Another object of the invention is to provide a card and method of theabove character in which the semiconductor device is relatively stressfree.

Another object of the invention is to provide a card and method of theabove character in which the semiconductor device can be readily andeconomically manufactured.

Additional objects and features of the invention will appear from thefollowing description in which the preferred embodiments are set forthin detail in conjunction with the accompanying drawings.

FIG. 1 is a plan view of the front side of the flexible electronic cardincorporating the present invention.

FIG. 2 is a plan view of the back side of the electronic card shown inFIG. 1.

FIG. 3 is a cross sectional view taken along the line 3--3 of FIG. 1.

FIG. 4 is a cross sectional view similar to FIG. 3 but showing the useof a semiconductor device which is embedded within the card.

FIG. 5 is a perspective view showing the manner in which a semiconductorwafer manufactured in accordance with the method of the presentinvention can be bent over a radius of 2" or less without breaking orfracturing.

FIG. 6 is a radial stress map of a wafer which has been ground inaccordance with the method of the present invention.

FIG. 7 is a graph showing the shape of a wafer utilized in the presentinvention before and after grinding.

In general, the flexible electronic card of the present invention is foruse with an electronic card reader. It is comprised of a flexiblesubstrate formed of a plastic and having dimensions such that it can fitinto a conventional billfold. A flexible semiconductor device carried bythe flexible substrate is accessible electronically by use of theelectronic card reader. The electronic card is characterized in that thecard and the semiconductor device carried thereby can withstand bendingover a 2" radius or less without breaking or damaging the semiconductordevice.

More specifically as shown in the drawings, the flexible electronic card11 consists of a flexible substrate formed of a suitable plastic as forexample polyethylene. It typically is sized so that it can fit within aconventional billfold. Thus it typically has a size having a length of33/8" and a width of 21/8" and a thickness of 30 to 32 mils. Inaccordance with the present invention, this thickness can range from 10to 40 mils. The plastic for making the card can be opaque or colored ifdesired. It is typically provided with many different types of indicia,some of which are visible to the human eye and some of which areinvisible to the unaided human eye.

The substrate is provided with front and back sides 13 and 14 which areplanar and parallel to each other. On the front side 13, there isprovided a rectangular space 16 which can be utilized for placing thename of the card issuer. A card number 17 is carried by the card 11which is visible to the user and typically is a number assigned to theuser and can be imprinted on the card or embossed onto the plastic ofthe card. The card also can carry a rectangular space 18 in whichcertain information can be carried, as for example the name of the userembossed in the card as well as valid dates for the card and otherdesired information. Another rectangular space 19 is provided on thecard which can carry additional identification, as for example the logoof the issuer as well as a holographic image to inhibit counterfeitingof the card. On the back side 14 of the card 11, a magnetic stripe 21extends across the card and incorporates therein certain magneticallyencoded information not visible to the user. It also includes arectangular space below the magnetic stripe 21 which carries a strip 23of the type which can be signed by the user of the card to provide anauthorized signature for checking use of the card. The remaining spaceon the back of the card can be utilized for carrying other printedinformation which the issuer may wish to place on the card.

A semiconductor device 26 made in accordance with the present inventionis carried by the flexible substrate 12 and as hereinafter explained canbe completely embedded in the card or can have a portion thereof exposedthrough either the front or back surface of the card. The semiconductordevice 26 carries means by which it can communicate with an electroniccard reader of a conventional type which can make electrical contactwith the card by physically making contact with a plurality of contacts27 (see FIGS. 1 and 3) which are carried by the semiconductor device 26and which are accessible through an opening 28 provided on the frontside 13 of the card 11. Alternatively as shown in FIG. 4, contact can bemade by an electronic card reader to the semiconductor device throughother communication means as for example a radio frequency antenna inthe form of a coil 31 carried by the semiconductor device 26 andembedded within the plastic forming the substrate 12.

In connection with the present invention, the semiconductor deviceutilized is of relatively large size to provide the desired electroniccapabilities for the device. Typically it will have a minimum dimensionranging from 100 mils by 100 mils to as large as 1,000 mills by 1,000mils.

In connection with the method of the present invention, thesemiconductor device is fabricated from a silicon semiconductor wafer asfor example having a diameter ranging from 4" to 8". Before utilizationof the method of the present invention, the semiconductor circuits to beutilized in the semiconductor device have been formed in the front sideof the wafer by conventional diffusion and evaporation processes wellknown to those skilled in the art and during which contact pads, as forexample the contact pads 27 hereinbefore described, have been providedon the front surface of the wafer.

Let it be assumed that the circuitry making up the semiconductor devicesformed in the wafer have been completed and that the wafer at this stageof processing has a thickness of 26 mils before back side grinding. Letit now be assumed that it is desired to thin the wafers by grinding theback side before dicing the wafers to form the desired semiconductordevices for the electronic card 11. Such thinner wafers are easier todice and have improved thermal dissipation. Typically in the past such agrinding process induced stress in the wafer and caused it to warp. Suchwarped wafers are more likely to break during dicing. Warped dice aremore difficult to mount and are prone to break and shatter particularlywhen they are flexed.

In accordance with the present invention in order to providesubstantially stress free wafers after back side grinding, the wafersare mounted face down on circular metal plates used in connection withthe grinder. These metal plates can have a suitable diameter, as forexample 101/2" and have a very planar surface. The wafer is mounted on aspinner. A lint-free laboratory paper (not shown) is then placed on theplanar surface of the mounting plate. The lint-free laboratory paper hasa suitable thickness, as for example 3 mils. Wax in the form of asynthetic resin dissolved in a liquid is poured into the center of thepaper. The spinner is then actuated to achieve a uniform distribution ofthe wax over the paper. Any excess is purged. The wax cover plate isthen heated until the liquid suspension solvent utilized in the wax hasbeen evaporated and all bubbles are eliminated, as for example heatingthe plate to a temperature of 150° C. After the metal plate has beenheated, the wafer with the circuit side down is placed over the waxedpaper. The metal mounting plate is then placed in a cold plate press.The wafers are engaged by a rubber backing plate to which a relativelyhigh pressure is applied, as for example 2500 lbs. of pressure on the 101/2" plate which by way of example can carry four 4" wafers. Thisensures that the wafers are firmly pressed against the mounting platewhich has a very flat parallel surface. The mounting plate during thispressing operation can be subjected to cooling to ensure that the waxhas solidified and holds the wafers firmly in place.

The mounted plates are then placed in a conventional grinder, as forexample a Blanchard grinder. Appropriate calculations are made todetermine the depth of the grind, taking care to include the thicknessof the lint-free paper and the wax embedded therein, as for example athickness of 3 mils to allow approximately 1/2 mil for polishing. TheBlanchard grinder is then automatically set to remove the desiredmaterial from the back side, as for example descending from 26 to 8 milsin thickness.

After the grinding operation has been completed, the mounting plateswith the wafers remaining mounted thereon are subjected to a polishingprocedure by the use of a conventional polisher, as for example oneapplied by Strausbaugh and a polishing slurry. As the polishingoperation continues, the thicknesses of the wafers are periodicallymeasured until the desired thickness has been reached. After the desiredthickness has been reached, the mounting plate with the wafers thereoncan be removed from the polisher. In connection with the present method,it should be appreciated that both the grinding and polishing operationshave been carried out on the wafer without removing the wafer or wafersfrom the mounting plate.

After the polishing has been completed, the mounting plates with thewafers thereon are placed on a hot plate until the temperature of thehot plate exceeds 150° C., at which time the wafers can be pried loosefrom the mounting plates utilizing tweezers. The wafers are placed in aTeflon boat. The wafers in the boat are introduced into a vapor zone ofa vapor degreaser for a suitable period of time as for example oneminute. The wafers are then rinsed in the liquid phase of the vapordegrester after which they are again subjected to another vapor zonetreatment and another liquid phase in the vapor degreaser. The wafersare thereafter drained and cooled. The wafers are then placed on avacuum chuck of a spinner. A cleaning solution is then introduced ontothe exposed surface of the wafer. While the wafer is spinning, a spongeis utilized to scrub the wafer after which the wafer is rinsed with DIwater and dried.

From the description of the method hereinbefore described, it can beseen that there is provided a "hot wax" method for securing the wafersduring the grinding and polishing process. No photoresist or passivationlayers are required to protect the surface of the wafers since thewafers are only mounted once on the mounting plate with the hot waxmethod. Thus, there is less wafer handling. As hereinafter explained,the method of the present invention makes it possible to produce waferswith back side grinding that are very thin and which have less stress bya factor of 10 than with conventional grinding and polishing methods.The method is also advantageous in that it tolerates the use of goldbumps and high ink dot materials on the semiconductor devices. Thepresent method can be utilized for grinding wafers from 8" (50-200millimeters) diameter to thicknesses down to 5 mils (1.25 millimeters)and even as low as 3 mils with a tolerance of plus or minus one-half mil(0.0125 millimeters).

After the grinding and polishing operations hereinbefore described havebeen completed, the wafers can be diced in a conventional manner toprovide individual dice which serve as semiconductor devices in thepresent invention. The semiconductor wafers that have been back groundin accordance with the present invention and before they are die cut canbe bent over a radius of 2" or less without fracturing or breaking asillustrated by the perspective view shown in FIG. 5. This is animportant characteristic of the wafers manufactured in accordance withthe present invention because it ensures that the die made therefrom arealso flexible and will not fracture or break when embedded in theflexible electronic card of the present invention.

A radial stress map of a wafer made in accordance with the presentinvention is shown in FIG. 6 and shows the stresses induced on a waferhaving a diameter of 158 millimeters with the analysis being done on aTencor FLX laser cantilever beam system at a viewing angle of 90°. Theunits on the map as shown in FIG. 6 are in MPa×μm(stress-times-thickness). The actual stress layer thickness is 10 μm andthe stress value in MPa is equal to the values in the map divided by 10.The wafer 41 shown in the drawing had thicknesses of 18 mils and adiameter of 158 millimeters. The wafer had the following statistics:

Film Thickness--10,000 A°

Average:+1.86

Minimum+27.32

Maximum-25.71

Standard Deviation: 3.84

Viewing Angle: 90°

From the order of stress for the wafer shown in FIG. 6 which has beenback-side ground in the manner hereinbefore described, had a scale forstress which is at least one order of magnitude lower than forconventional wafers which have been back-side ground. In addition, therelative smoothness of the sawed edge of the polished wafers which havebeen back side ground and polished in the manner hereinbefore describedis less than 2 microns peak-to-peak versus 4+ microns peak-to-peak forprior art methods. The radial stress map shown in FIG. 6 shows that atleast 80% of the die cut from the wafer will be relatively stress freein comparison to approximately 30% for wafers which have their backgrinding accomplished by conventional back grinding methods.

In FIG. 7 there is a graph showing deflection of a wafer ground inaccordance with the present invention from 26.5 mils to 18 mils. Thedeflection is measured from a three-point support for the wafer at itsouter periphery. The solid line 51 represents deflection of the wafer at26.5 mils in thickness before grinding due to its self weight withdeflection being approximately 5 μm whereas the solid line 52 representsdeflection of the wafer due to its self weight after grinding thethickness down to 18 mils with the deflection being approximately 12micrometers. In ascertaining these deflections, the deformation of thesubstrate was ascertained by a film deposited thereon to measure thefilm stress. As is well known to those skilled in the art, stresses canbe calculated by equating the forces and moments of the film and thesubstrate.

From the foregoing, it can be seen that a method has been provided inthe present invention which results in much higher die yields per waferand therefore makes it possible to produce semiconductor die at lowerprices to result in dramatic cost savings. In addition, the devices madein accordance with the invention are relatively stress free andtherefore can be bent through a substantial angle as for example anangle corresponding to a 2" radius without fracturing or breaking.Utilizing the same principles is possible to manufacture die having athickness from 2 to 7 mils and certainly within 4 to 5 mils so that theycan be readily encapsulated in flexible plastic substrate of the typehereinbefore described to provide the flexible electronic card of thepresent invention. The semiconductor device utilized can have largestorage capabilities and large computation capabilities. Since thewafers made in accordance with the present invention are relativelystress free, it dices very well and the resultant die are tough enoughto withstand extreme punishment when embedded in a plastic substrateutilized for flexible electronic cards. Thus for example, suchelectronic cards can be carried in a billfold and can withstand repeatedbending which can occur in a billfold carried in the back pocket of thepants of a wearer.

What is claimed is:
 1. A flexible electronic card for use with anelectronic card reader comprising a flexible substrate formed of plasticand having dimensions of approximately 33/8" by 21/8" such that it canfit into a conventional billfold, a flexible semiconductor device havingan area greater than 100 mils by 100 mils carried by the flexiblesubstrate; and being accessible electronically by the electronic cardreader characterized in that the; card and the semiconductor devicecarried thereby can withstand bending over a 2" radius without breakingor damaging the semiconductor device, said semiconductor device havingedges and having a substantially uniform thickness in the range of 2 to7 mils and being free of tapered edges, said semiconductor device havinga back surface; that is ground and polished with peak-to-peak variationsof less than 2 microns to provide a semiconductor device withsubstantially reduced stress.
 2. A card as in claim 1 wherein saidsemiconductor device has a thickness from 3 to 5 mils.
 3. A card as inclaim 2 wherein said semiconductor device has an area as great as 1000mils by 1000 mils.
 4. A card as in claim 1 wherein said card has athickness of 30-32 mils.
 5. A card as in claim 1 wherein saidsemiconductor device carries contact pads and wherein said substrate isprovided with an opening overlying the contact pads so that the contactpads are accessible to the electronic card reader.
 6. A card as in claim1 wherein said card is embedded in the said plastic substrate andwherein said card is provided with a radio frequency antenna forconveying information by radio frequency from and to the electronic cardreader.
 7. A card as in claim 1 wherein said semiconductor device issubstantially stress free.
 8. A flexible electronic card for use with anelectronic card reader comprising a flexible substrate formed ofplastic, a flexible semiconductor device carried by the flexiblesubstrate and being accessible electronically by an electronic cardreader characterized in that the semiconductor device has edges and hasa substantially uniform thickness free of tapered edges in the range of2 to 7 mils and has been selected from a semiconductor wafer havingfront and back sides with a plurality of semiconductor devices in thefront side and in which the back side has peak-to-peak variations ofless than 2 microns treated by the steps of providing a mounting platehaving a planar flat surface, placing lint-free tissue paper on theplanar flat surface, placing a wax on the lint-free paper and spinningthe metal plate to cause a uniform distribution of the wax in thelint-free paper, placing the front side of the wafer against thelint-free paper, compressing the wafer against the lint-free paper whilesubjecting the mounting plate to heat in excess of 250° C., coating themounting plate grinding the back side of the wafer while it is supportedby the mounting plate to reduce the thickness of the semiconductor waferto a predetermined thickness, polishing the back side after the grindingoperation has been utilizing the same mounting plate without removingthe wafer from the mounting plate to further reduce the thickness of thewafer to a predetermined thickness, heating the mounting plate, removingthe wafer from the mounting plate, removing the wax from the frontsurface of the wafer, cleaning the wafer, die cutting the wafer toprovide an individual semiconductor die for use on the substrate toprovide an electronic card which can withstand extreme punishment andwhich can withstand bending over a 2-inch radius or less withoutbreaking or damage to the semiconductor device.